Enteroaggregative Escherichia coli (EAEC) ranks as a major pathogen of endemic and epidemic diarrhea in both resource-poor and resource-rich countries, and is a cause of traveler's diarrhea and persistent diarrhea in HlV-infected patients. We have found that the AraC-class transcriptional activator AggR is a global regulator of EAEC virulence factors. We have accordingly defined "typical" EAEC as those strains possessing the AggR regulon, and have shown that they are epidemiologically linked to diarrhea. AggR controls expression of Aggregative Adherence Fimbriae, a protein capsule that modulates adherence, a secretion system for the capsule monomer, a chromosomal pathogenicity island, and AggR itself. The central hypothesis of this application is that the AggR regulon acts throughout EAEC infection, maintaining expression of several virulence factors. This application will comprise three aims. Aim 1: Inflammatory factors in EAEC pathogenesis. Our data suggest that non-flagellar factors may induce cytokine release by EAEC-infected intestinal epithelial cells. In this aim, we will identify non-flagellar factor(s) that contribute to EAEC-induced inflammation, hypothesizing that one or more such factors are under AggR control. We will also address a possible role for Cox-2 in EAEC pathogenesis. Aim 2: Characterization of AggR regulation. Preliminary data suggest that aggR regulation is complex, involving the FIS protein and AggR itself binding to two sites flanking PaggR. Our data also suggest that certain commensal bacteria may influence aggR expression via heterologous quorum sensing. We will ask a) What is the role of FIS in aggR regulation? b) What is the role of heterologous quorum sensing in aggR regulation? and c) Does the downstream AggR binding site affect in vivo expression of aggR? Aim 3: Understanding the AggR regulon. It is as yet unclear why the AggR regulon correlates with diarrhea in epidemiologic studies. In this aim, we will take a genomics approach to look for additional AggR-regulated genes, and characterize expression of the AggR regulon in vivo.